1. Field of the invention
The present invention relates to an optical component using an optical material which can be adjusted to have desired characteristics.
2. Related Background Art
An optical component made up of an optical waveguide and the like undergoes a change in optical path length with a change in temperature T, resulting in a change in characteristics. The rate of change in optical path length is the sum of a temperature dependence dneq/dT of an effective refractive index neq and a thermal expansion coefficient xcex1sub. It is generally preferable that an optical component keep its characteristics constant at any temperature within an operating temperature range. In order to eliminate such a temperature dependence, a plurality of materials whose refractive index temperature coefficients and thermal expansion coefficients differ in sign must be selected and an optical component must be formed by using the selected materials such that the optical component satisfies the athermal condition represented by the following equation within an operating temperature range as a whole (e.g., Yasuo Kokubun, xe2x80x9cTemperature-independent lightwave devicesxe2x80x9d, OYO BUTURI, Vol. 66, No. 9, pp. 933-938 (1997)).                                                         1                              n                eq                                      ⁢                          xe2x80x83                        ⁢                                          ⅆ                                  n                  eq                                                            ⅆ                T                                              +                      α            sub                          =        0                            (        1        )            
For example, as a temperature dependence eliminating (temperature compensation) technique for an optical fiber grating element serving as an optical component having a Bragg grating formed in the core region of an optical fiber, a technique of mounting an optical fiber grating element on a member having a negative thermal expansion coefficient (e.g., silica glass, liquid crystal polymer, bimetal, glass, metal, or the like) is known (e.g., G. W. Yoffe, et al., xe2x80x9cTemperature-compensated optical-fiber Bragg gratingsxe2x80x9d, OFC ""95 Technical Digest, WI4 (1995)). According to this technique, this mount member gives the optical fiber grating element a negative thermal expansion (xe2x88x921xc3x9710xe2x88x925/xc2x0 C.) to cancel out the positive refractive index temperature dependence (1xc3x9710xe2x88x925/xc2x0 C.) of silica glass as the main constituent of the optical fiber, thereby satisfying the athermal condition represented by equation (1).
An organic material often has a negative refractive index temperature coefficient and a large positive thermal expansion coefficient. In terms of the principle of temperature dependence elimination, this material can be regarded as a material that exhibits a great decrease in refractive index due to a density decrease caused by thermal expansion as compared with an increase in refractive index due to electronic polarization caused by a temperature rise. On the other hand, an inorganic material such as silica glass has a positive refractive index temperature coefficient. Studies have therefore been made on a technique of satisfying the athermal condition represented by equation (1) by forming an optical component using a combination of an organic material and inorganic material (silica glass).
For example, an optical waveguide which is one type of optical component, in which a core region is made of an inorganic material (SiO2xe2x80x94GeO2), a cladding region is partly made of an inorganic material (SiO2), and the remaining part is made of organic materials (PMMA and TFMA) (e.g., shigeru Yoneda, et al., xe2x80x9cDesign of silica-based athermal optical waveguidexe2x80x9d, PROCEEDINGS OF THE 1997 IEICE GENERAL CONFERENCE, C-3-2, p. 187 (1997))is known. According to this technique, the overall temperature dependence of the optical component is eliminated by using the difference in the temperature dependence of refractive index between light in the core region and light entering the cladding region.
Likewise, an AWG (Arrayed Waveguide Grating) which is a kind of optical component and, in which most of the optical path of the arrayed waveguide grating in the longitudinal direction is made of an inorganic material (silica-based material), and part of the optical path is made of an organic material (silicone resin) (e.g., Y. Inoue, et al., xe2x80x9cAthermal silica-based arrayed-waveguide grating (AWG) multiplexerxe2x80x9d, IOOC-ECOC97, pp. 33-36 (1997)) is known. This technique is designed to eliminate the overall temperature dependence of the optical component by making the temperature dependences of the inorganic material and organic material on the optical path cancel out each other.
The following problems arise in the above mentioned temperature dependence eliminating technique. When an optical component is mounted on a member having a thermal expansion coefficient different in sign from that of the optical component, the mounting step is required in addition to the steps of fabricating the optical component. The arrangement including this member is not simple. When part of the cladding region of an optical component is made of an inorganic material and the remaining part is made of an organic material, it is difficult to make the overall cladding region uniform in material properties, and hence the optical component is likely to have polarization characteristics. In addition, When part of an optical path extending in an optical component in the light propagating direction is replaced with an organic material, a great optical loss is caused by, for example, reflection at the interface between the inorganic material and the organic material.
The above description is associated with the temperature dependence of an optical component whose temperature dependence is preferably eliminated or reduced. Some optical components used as active devices having the function of controlling characteristics by temperature adjustment preferably have high temperature dependence. In this case as well, an arrangement similar to that described above is conceivable, which suffers problems similar to those described above.
The present invention has been made to solve the above mentioned problems. Objects of this invention are to provide an optical component which has a low optical loss, has a simple arrangement, can be easily manufactured, and has desired temperature characteristics.
An optical component according to the present invention is characterized in that an optical material obtained by chemically bonding an inorganic material containing SiO2 as a main constituent and an organic material or by mixing a particulate inorganic material and an organic material is used for an optical waveguide region. The organic material is preferably silicone resin. The optical material obtained by chemically bonding the inorganic material and the organic material is preferably ladder-type silicone resin. In accordance with the mixing ratio between the inorganic material and the organic material, the optical material has a desired temperature dependence of refractive index. In addition, in accordance with a combination of the above optical material and another material, the optical component according to the present invention has a desired thermal expansion coefficient and desired temperature characteristics as a whole. Therefore, this optical component need not use another member for mounting. This facilitates the manufacturing process and allows a simple arrangement. In addition, in the optical component, the optical material can be made uniform in material properties along the optical path of the optical waveguide, thereby attaining a reduction in optical loss.